1. Field of the Invention
The present invention is generally in the field of electrical circuits and systems. More particularly, the invention relates to circuits for generating oscillating signals.
2. Background Art
Oscillation generators generate an oscillating signal, which can be used in various electronic circuits. For example, voltage-controlled oscillators (VCOs) and the like are commonly included in radio-frequency (RF) modules to generate an oscillating signal. A VCO can include a tank circuit (e.g., an LC circuit) and a gain stage for generating and amplifying the oscillating signal. The gain stage typically includes output transistors to supply a bias current from a bias current source to the tank circuit. It is desirable to implement the RF module using advanced complementary metal-oxide-semiconductor (CMOS) processes. For example, by using both PMOS and NMOS output transistors, a CMOS VCO, such as a CMOS complementary VCO (i.e. a CMOS VCO based on a complementary cross-coupled structure), can generate an oscillating signal with the same amplitude as an NMOS-only LC-VCO using half as much bias current.
However, the bias current source and the gain stage of the oscillation generator require voltage headroom for start up so as to reliably start generating the oscillating signal with the tank circuit. Furthermore, the bias current source and the gain stage require voltage headroom to ensure that they operate in saturation mode in order to maintain good supply rejection. These voltages introduce challenges when implementing the VCO with a low supply voltage. For example, while a low voltage supply, such as 1.2 volts, may be sufficient to satisfy voltage headroom requirements to start up a tank circuit of an NMOS-only LC-VCO, the gain stage of a CMOS complementary VCO may require a higher supply voltage to start up due to having additional output transistors in its gain stage. It has been contemplated that a static bias voltage could be applied to gates of output transistors in the gain stage of the CMOS complementary VCO to provide sufficient voltage headroom for start up of its tank circuit. However, the static bias voltage would undesirably reduce an amplitude of oscillations produced by the CMOS complementary VCO. Among other disadvantages, this increases phase noise of the oscillating signal.
Thus, it would be desirable in the art to provide oscillation generators that can satisfy voltage headroom requirements for startup of a tank circuit without reducing an amplitude of oscillations produced by the oscillation generator.